In recent years, in an image forming apparatus (for example, an MFP), a sheet finishing apparatus is provided adjacently to a latter stage of the MFP in order to perform finishing on a sheet after image formation. The sheet finishing apparatus is called a finisher, and punches a punch hole in the sheet sent from the MFP or staples it. Or, in some finishers, a sheet bundle is folded and is discharged.
In a finisher including a sheet folding unit, after a punch hole is formed, folding is performed. When a sheet bundle becomes thick, since the position of the punch hole shifts between a sheet at the front cover side and a sheet at the inside, the punch position is shifted between the front cover side and the inside.
JP-A-2000-1256 discloses a punch apparatus. In this example, punch holes are punched symmetrically with respect to the center position of a sheet, an interval between the punch holes is made small for the sheet at the inside of a sheet bundle, and an interval between the punch holes is made large for the sheet at the front cover side.
However, in the punch apparatus as stated above, a sheet can move obliquely (hereinafter called skew) with respect to a conveyance direction of a sheet, and when punching is performed while the skewing is occurring, the punching position is obliquely shifted, and a trouble occurs when filing is performed.